Just type these codes in at the title screen after
codes are enabled. To enable codes you must:

1. Place first on the full ciciut on Intermediate.
This will allow you to access 4 new tracks
2. Place first on the full circuit again this time on Professional.
This will enable 2 more tacks
3. Place first on the full circuit again on Professional.
This will enable the final track.
4. Place first on the full circuit again on Professional

Now when you go back to the title screen, the rider in the middle
should have a cartoon bubble near her that says "Codes Enabled"
Now you can use the codes.